Assumptions can simplify the complex world and make it easier to understand. To study the effects of international trade, for example, we might assume that the world consists of only two countries and that each country produces only two goods. In reality, there are numerous countries, each of which produces thousands of different types of goods. But by assuming two countries and two goods, we can focus our thinking on the essence of the problem. Once we understand international trade in this simplified imaginary world, we are in a better position to understand international trade in the more complex world in which we live. The art in scientific thinking—whether in physics, biology, or economics—is deciding which assumptions to make.

This doesn’t mean we have to go around in circles, though. Zeilinger tells me an instructive story. While a college student, he and a cousin backpacked around France on motor scooters. One night they pulled up to a youth hostel recommended by their guidebook, only to find it closed and abandoned. They decided to sneak in and sleep there anyway. As they settled in for the night, Zeilinger opened a window for air. A short while later, his cousin got up to close it. A while later, Zeilinger reopened it. And so it went all night. “I was convinced it was stuffy; he was convinced it was drafty,” Zeilinger remembers. Only in the morning did they notice that the window didn’t have any glass in it. Their nocturnal passive-aggressiveness had been pointless. For Zeilinger, the moral is that you always need to question your own assumptions.

George Musser, Spooky Action at a Distance (2015), Chap. 4 : The Great Debate